The invention relates generally to the field of marine seismic surveying. More particularly, the invention relates to seismic energy sources used in such surveying.
Marine seismic surveying may be used to infer structure and composition of formations below the bottom of a body of water such as a lake or ocean. Marine seismic surveying typically includes towing a seismic energy source using a survey vessel or another vessel, or an array of such sources, in the body of water, and actuating the source at selected times. One or more seismic streamer cables may be towed by the survey vessel or by another vessel. The seismic streamer cables include seismic sensors at spaced apart locations, which detect seismic energy that may be reflected from acoustic impedance boundaries in the formations below the water bottom. Cables having seismic sensors may also be deployed on the water bottom.
Seismic energy sources known in the art for marine seismic surveying include air guns and water guns. Such seismic energy sources may include pressure chambers to hold compressed gas, air or water. The compressed gas, air or water may be released by selective operation of a control valve such that rapid discharge of the compressed gas, air or water creates a pulse of seismic energy in the body of water.
Other seismic energy sources known in the art include marine vibrators. An example of a marine vibrator is described in U.S. Pat. No. 7,551,518 issued to Tenghamn, which is incorporated herein by reference.
A need has been identified for marine seismic energy sources that can generate substantial energy, at frequencies less than about 10 Hz. 10 Hz is believed to be the practical lower frequency limit of air guns, water guns and marine vibrators known in the art. One of the limiting factors for air guns and water guns with respect to a lowest practical output frequency is the size of the chamber used to store the pressurized gas, air or water. One of the limiting factors of a seismic energy source such as a marine vibrator is the power required to move the driving element at low frequencies and to maintain a reasonably pure tone, because hydraulically driven devices in the vibrator may have difficulty shifting the required fluid volume for such low frequencies. Thus, the need for a seismic energy source having output in such frequency range has yet to be adequately addressed.